1. Field of Invention
This invention relates to a new coating composition and a method for applying it to convert a corroded steel surface into a suitable substrate for accepting organic coatings. More specifically, it relates to an improved composition and a related method for treating corroded steel surfaces at stage 1 (mild corrosion) and/or stage 2 (pitted corrosion) to render them suitable, without the need for sandblasting, for the application of an organic primer coat.
2. Description of Prior Art
Many methods for protecting steel surfaces exposed to air and water, including salt water, are currently in use. Generally, before a primer or final protective paint coating is applied to a corroded steel surface, it must be cleaned or prepared by sand blasting to the bare metal. After sandblasting, the surface must be degreased. However, for some steel objects, such pretreatment is impractical for environmental, structural, safety and cost reasons. The treatment of mildly corroded steel surfaces without prior full mechanical removal of the surface oxide layer by sandblasting therefore is a most useful technique.
A well-known process for treating steel surfaces before the application of primer and final paint coatings is phosphating, a procedure by which a water-insoluble film of tertiary ferric phosphate is formed on the steel surface by treatment with phosphoric acid or acid phosphate salts. This film is a suitable substrate for primer and final paint coatings.
Also used for the preparation of steel surfaces are tannins and their derivatives. Tannins also are used in combination with phosphoric acid or acid phosphate salts. U.S. Pat. No. 2,502,441, for example, uses an acid phosphate salt, a molybdate or tungstate salt, tannin or catechol, and a wetting agent. U.S. Pat. No. 2,854,368 discloses the use of phosphoric acid and tannin and an alcohol, U.S. Pat. No. 4,293,349 discloses the use of phosphoric acid, zinc or manganous phosphate and nitrates and tannin derivatives.
None of the above combinations of ingredients have proved entirely satisfactory for treating corroded steel exposed to marine environments. Thus, there is a need for a novel composition and method for treating corroded steel surfaces to convert them into substrates suitable for accepting organic coatings without the need for prior sandblasting, specifically for use in a marine environment.